In power processing circuits, magnetic elements and capacitors invariably play the major roles of energy storage, energy transfer, and ripple filtering. Since they constitute a large portion of the overall equipment, weight, volume, and cost, it is desirable to design a converter capable of operating at high frequencies. However, switching an inductive load at high frequencies imposes high switching losses and switching stresses on semi-conductor devices.
Operation of converters at megahertz frequencies is also strongly influenced by the effects of such parasitic elements as interconnect and leakage inductance and parasitic and junction capacitances. Some parasitics, such as leakage inductance of a transformer, can be constructively used in the circuit. Other parasitics may have adverse effects on circuit performance and may be minimized by a compact circuit layout design.
Resonant converters have been found to be attractive for high-power applications because they allow high frequency operation and reduction in size and weight, without sacrificing circuited efficiency and without imposing excessive stress on switching devices.
In a parallel resonant converter (PRC), the output voltage is obtained after rectification and filtering of the resonant capacitor voltage, whereas in a series resonant converter (SRC) the output voltage is obtained after rectification and filtering of the resonant current. Generally, when operating at frequencies that are sufficiently less than the tank frequency, a SRC behaves as a current source, whereas a PRC acts as a voltage source. Thus, in voltage regulator applications, a PRC requires much less operating frequency range than an SRC to compensate for load variations. A PRC is preferable for voltage regulation applications having a wide load variation, such as switching power supplies, mainly because of its load independent feature. Besides being relatively load insensitive, a PRC has excellent control characteristics and provides a good cross regulation compared to a SRC. Other advantages of a PRC include: low losses during switching, low EMI, low switching component stresses, low frequency control range, high frequency operation, helpful parasitics, and simple control loop compensation. The disadvantages of a PRC are: high complexity (relative to a PWM type switchmode power supply), high circulating energies and secondary current limiting. On the whole, a PRC is often the preferred configuration. A detailed graphical analysis of a PRC is described in a 1985 paper, "State-Plane Analysis of a Parallel Resonant Converter", by Oruganti and Lee (IEEE Catalog No. 0275-9306/85/0000-0056).
Those skilled in the art know that a converter operating in the continuous conduction mode neither has the voltage across the capacitor or the current through the inductor staying zero for any time interval. A half-bridge PRC is analyzed in detail for both continuous conduction mode and discontinuous conduction mode operations in a 1987 paper, "Analysis and Design of a Half-Bridge Parallel Resonant Converter", by Kang and Upadhyay (IEEE Catalog No. 0275-9306/87/0000-0231).
The DC to DC conversion ratio "M" of resonant converters is often controlled by changing the ratio of switching frequency to resonant frequency "f.sub.s /f.sub.0 ". An analysis is provided in the paper, "Small Signal Analysis of Resonant Converters", by Cuk et al, IEEE Power Electronics Specialists Conference, June 6-9, 1983.
Pulse Width Modulation (PWM) is frequently employed in the control of switch mode power supplies. The PWM technique processes power by interrupting the power flow and controlling the duty cycle; thus, pulsating current and voltage waveforms result. By contrast, the resonant control technique processes power in a sinusoidal form.
For a given switching converter, the presence of leakage inductances in the transformer and junction capacitances in semi-conductor devices, causes the power devices to operate in inductive turn-off and capacitive turn-on. When a semi-conductor device switches off an inductive load, voltage spikes are induced by the sharp di/dt across the inductances. On the other hand, when the device turns on at a high voltage level, the energy stored in the output capacitance, 0.5 CV.sup.2, is trapped and dissipated inside the device. Furthermore, turn-on at high voltage levels induces a severe switching loss, known as the Miller Effect, which is coupled into the drive circuit, leading to significant noise and instability. The capacitive turn-on loss due to the discharging of the parasitic junction capacitances of power MOSFETs often becomes the dominating factor when the switching frequency is raised to the megahertz range.
The concept of zero-current switching is disclosed in U.S. Pat. No. 4,720,667 to Lee et al. However, the zero-current switching technique cannot solve the problem of high switching loss associated with the capacitive turn-on; therefore, its operation is somewhat limited to the lower megahertz range. U.S. Pat. No. 4,720,668 to Lee et al, discloses a zero-voltage switching technique. Specifically, a family "quasi-resonant converters" is derived from the principal of the zero-current switching converters by applying the duality principle. For the zero-current switching technique, the objective is to use auxiliary LC resonant elements to shape the switching device's current waveform at on-time in order to create a zero-current condition for the device to turn-off. The dual of this objection is: To use auxiliary LC resonant elements to shape the switching device's voltage waveform at off-time in order to create a zero-voltage condition for the device to turn-on. Further information is given in the 1986 paper, "Zero-Voltage Switching Technique in DC/DC Converters", by Liu and Lee, (IEEE Catalog No. 0275-9306/86/0000-0058). Thus, PRC's and methods to control them are still not fully developed and further work is needed.